The present invention relates to a roll electrode and a device for treating the surfaces of film webs by means of an electrical corona discharge. The roll electrode of the invention comprises an electrically conductive support roll and at least one dielectric layer applied to this support roll.
In the fields of converted plastic sheets and composite films, surface treatment by means of an electrical corona discharge is known in order to render the surface printable or to increase the bond strength of several layers. In this treatment, the plastic sheet or film is passed over an electrically grounded support surface, usually a roll, and the side of the film which faces away from the support surface is exposed to an electrical corona discharge which is generated by supplying high-frequency, high-voltage A.C. to an electrode arranged at a distance from the support surface. The known devices operating according to this basic principle differ only in the construction and the materials of the support surface serving as the counter-electrode (for example, an individual central roll with peripherally arranged electrodes or several electrode rolls with their respective electrodes), in the dielectric materials used to insulate the counter-electrode, in the design of the electrode used, and in the type of generator employed in each case.
As the simplest and thus the favored design of the roll electrode, metal support rolls comprised of a solid material, in particular, steel or aluminum rolls, coated with layers of insulating materials such as, for example, glass, ceramics, enamel, rubber, or glass fiber reinforced plastics, have gained general acceptance. This construction principle has the disadvantages that equipment costs are considerably increased since expensive steel rolls must be provided and that the roll weights are so high that, particularly in large-scale installations, technical problems arise in connection with the bearing, bending, true running and driving of the rolls. To overcome these problems, U.S. Pat. No. 4,239,973 and European Patent No. 0 086 977 describe corona devices which use hollow roll bodies comprising fiber-reinforced synthetic resin tubes. These fiber-reinforced tubular bodies, which have only a fraction of the weight of steel rolls, not only meet mechanical requirements, but naturally also act as the electrodes due to the wire winding embedded in the synthetic resin matrix. The manufacture of these glass fiber-reinforced roll bodies does not present any difficulty whatsoever; however, it appears that, upon embedding a wire winding or a wire helix into the synthetic resin matrix to render it electrically conductive, the interlaminar shear strength, that is, the adhesion between the synthetic resin matrix and metal wire, leaves much to be desired.